CSP-D507: TOCTOU Race Condition¶

Vulnerability Category: Filesystem

Severity: MEDIUM

Description¶

This rule flags filesystem time-of-check/time-of-use patterns, such as checking os.path.exists() or Path.exists() before opening the same path.

Between the check and the use, another process can replace the file, swap a symlink, or alter permissions. This can lead to reading or writing the wrong file, overwriting sensitive data, or bypassing intended checks.

Vulnerable Code Example¶

import os

def read_config(path):
    if os.path.exists(path):
        with open(path) as handle:
            return handle.read()
    return ""

The file can change after exists() returns but before open() runs.

Safe Code Example¶

def read_config(path):
    try:
        with open(path) as handle:
            return handle.read()
    except FileNotFoundError:
        return ""

Use the operation directly and handle the exception. For writes, use atomic creation modes and secure file APIs where possible.

How to Suppress a Finding¶

Suppress only when the path is not attacker-influenced and the race cannot cross a security boundary.

# path is an internal temporary file in a private directory.
# ignore: CSP-D507
if os.path.exists(path):
    open(path).close()